Based on a Tier Ⅲ marine low-speed two-stroke diesel engine,the effects of load, EGR rate and fuel injection parameters on the heat release rate and its characteristic parameters are studied. The research shows that the load and rail pressure are the main factors affecting the start of combustion,load and higher rail pressure lead to earlier combustion starting point; higher rail pressure and earlier injection timing lead to earlier CA50 so that the indicated thermal efficiency is increased; small EGR rate has less influence on the combustion process until EGR rate is higher than 26%, at which point CA50 is significantly postponed, the combustion duration is increased, and the indicated thermal efficiency is lowered. Based on mesured heat release rates, three-Wiebe function was studied, the influence rule of multi-factors on heat release empirical model was discovered, and the empirical model suitable for low-speed engines was obtained.